The cause of liver failure in cirrhosis is not well understood, as the liver is capable of regeneration and functions normally despite loss of more than half of its hepatocytes. Hepatic failure in cirrhosis therefore results from additional dysfunction of the remaining hepatocytes. Our Preliminary Studies showed that primary hepatocytes derived from cirrhotic livers with decompensated function exhibit numerous alterations in gene expression and proliferative capacity. Yet, despite critical shortening of telomeres and loss of telomerase activity, these hepatocytes can eventually recover their capacity for regeneration and function after transfer into a normal liver. Our transcriptome analysis demonstrated that progression to hepatocyte failure was associated with down regulation of transcription factor HNF4? and suppression of its regulatory network. Since HNF4? deficiency could explain the depression of downstream effectors, mature hepatocyte-specific genes, and general hepatic function, we studied end-stage cirrhotic hepatocytes in culture and found that HNF4? expression, restored via AAV transduction, dramatically and immediately corrected the phenotype. Next, AAV-HNF4? was given IV to rodents with decompensated cirrhosis. Their hepatocyte function improved to almost normal levels within 2 weeks and survival was prolonged from ~2 weeks to more than 100 days! Thus, disruption of HNF4? transcriptional activation appears to be the mechanism responsible for hepatic failure in advanced cirrhosis. We therefore hypothesize that restoration of HNF4? will effectively treat cirrhotic patients with end-stage liver failure by correcting the metabolic defects and reversing the hepatocyte replicative senescence. In these studies we will use a recombinant AAV vector that encodes HNF4? driven from an inducible promoter to determine whether short term treatment with HNF4? can lead to sustained normalization of hepatic function and survival. In addition, we will assess the utility of HNF4?-AAV treatment during continuing injury with CCL4 to determine the potential clinical efficacy of such therapy. To determine the mechanism of HNF4? downregulation in decompensated cirrhosis, we will perform a genome-wide ChIP-Seq analysis of isolated hepatocytes and whole liver. Finally, we will determine the extent to which human end-stage cirrhotic livers share the same characteristics as those identified in our rodent studies, and will determine the extent that restoration of HNF4? expression can normalize human hepatocytes derived from these end-stage livers.